tZo OF VERTIGO. Sect. XX. 6V 



to be inconfcious, Dr. Wells afcribes the apparent circumgyra- A 

 tion of objects on ceafing to revolve. 



The caufe of thus turning our eyes forwards, and then back 

 again, after our body is at reft, depends, I imagine, on the fame 

 nircumllance, which induces us to follow the indiftinct. fpectra, 

 which are formed on one fide of the centre of the retina, when 

 we obferve them apparently on clouds, as defcribed in Seel. XL. 

 2. 2 ; and then not being able to gain a more diftinct vifion of 

 them, we turn our eyes back, and again and again purfue ffie fly- 

 ing (hade. 



But this rolling of the eyes, after revolving till we become 

 vertiginous, cannot caufe the apparent circumgyration of objects, 

 in a direction contrary to that in which we have been revolv- 

 ing, for the following reafons. I. Becaufe in purfuing a fpec- 

 trum in the fky, or on the ground, as above mentioned, we per- 

 ceive no retrograde moti&ns of objects. 2. Becaufe the appar- 

 ent retrograde motions of objects, when we have revolved till 

 we are vertiginous, continues much longer than the rolling of 

 the eyes above defcribed. 



3. When we have revolved from right to left, the apparent 

 motion of objects, when we (lop, is from left to right •, and 

 when we have revolved from left to right, the apparent circula- 

 tion of objects is from right to left : yet in both thefe cafes the 

 eyes of the revolver are feen equally to roll forwards and back- 

 wards. 



4. Becaufe this rolling of the eyes backwards and forwards 

 takes place during our revolving, as may be perceived by the 

 hand lightly p'reffed on the clofed eyelids, and therefore exifts 

 be r ore the effect afcribed to it. 



And fifthly, I now come to relate an experiment, in which the 

 rolling of the eyes does not take place at all after revolving, and 

 yet the vertigo is more diftreflmg than in the fituations above 

 mentioned. If any one looks fleadily at a fpot in the ceiling 

 over his head, or indeed at his own finger held up high over his 

 'head, and in that fituation turns round till he becomes giddy ; 

 and then flops, and looks horizontally *, he now finds, that the 

 apparent rotation of objects is from above downwards, or from 

 below upwards ; that is, that the apparent circulation of objects 

 is now vertical iidlead of horizontal, making part of a circle round 

 the axis of his eye ; and this without any rolling of his eyeballs. 

 The re.;fon of there being no rolling of the eyeballs perceived 

 after this experiment, is, becaufe the images of objects are form- 

 ed in rotation round the axis of the eye, and not from one fide 

 •;o Cac other of the axis of it ; fo that, as the eyeball has not 

 power to turn in -its focket round its own axis, it cannot follow 



• the 



